Known nitride-applied optic and electronic devices are manufactured on sapphire or silicon-carbide substrates, differing from the deposited nitride layers (i.e. heteroepitaxy). In the most commonly used MOCVD method GaN is grown by a vapor phase epitaxy from ammonia and metallo-organic compounds, but it is impossible to get a bulk monocrystalline. In the present situation, by using a buffer layer the number of dislocation per unit area can be reduced, however the reduction of surface dislocation density achieved is up to about 108/cm2.
Recently, the dislocation density can be decreased by using the Epitaxial Lateral Overgrowth (ELOG) method. In this method, a GaN layer is first grown on the sapphire substrate, and then SiO2 is deposited in the form of strips or grids. Next, such a substrate may be used for ELOG of GaN, thereby reducing the dislocation density below about 107/cm2. However, this method has a difficulty in reducing the dislocation density 106/cm2 or less.
In the HNP method [“Prospects for high-pressure crystal growth of III-V nitrides” S. Porowski et al., Inst. Phys. Conf. Series, 137, 369 (1998)] growth of crystals is carried out in melted gallium, i.e. in the liquid phase, however resulting in production of GaN crystal about 10 mm in size. Sufficient solubility of nitrogen in gallium requires temperatures of about 1500° C. and nitrogen pressures of the order of 15 kbar. In another known method, supercritical ammonia was proposed to lower the temperature and decrease the pressure during the growth process. [“ammono method of BN, AlN, and GaN synthesis and crystal growth” R. Dwilinski et al., Proc. EGW-3, Warsaw, Jun. 22-24, 1998, MRS Internet Journal of Nitride Semiconductor Research] [“Crystal Growth of gallium nitride in supercritical ammonia”, J. W. Kolis et al., J. Cryst. Growth 222, 431-434 (2001)]. However, only GaN crystal having about 0.5 mm length and no bulk monocrystalline was obtained, therefore such GaN crystal cannot be used as a substrate of electrical devices etc.